Amorphous diamond films (also known in the literature as highly tetrahedral amorphous carbon (ta--C), amorphic diamond.TM., or amorphous carbon, a--C,) is an amorphous form of carbon that has a high fraction (&gt;.about.50%) of 4-fold coordinated carbon atoms (similar to the bonding found in crystalline diamond) with the rest of the carbon atoms being 3-fold coordinated (similar to the bonding found in graphite). These amorphous diamond films are deposited by pulsed laser deposition (PLD) using a solid graphite target and (typically) an excimer laser, cathodic or filtered arc deposition using graphite electrodes, or mass-selected carbon ion deposition. The key requirement for the formation of amorphous diamond films is that the film is produced, at least in part, by the deposition of carbon ions with high energies (typically, 10 to 100 eV). These films are transparent, insulating, extremely hard, and contain negligible (&lt;0.1%) amounts of hydrogen. These films also have very high (&gt;6 GPa) compressive stress which introduces severe limitations for their application: (1) it limits the film thickness which can be deposited (to typically &lt;1500 .ANG.); (2) it causes problems with adhesion of the films on various substrates; (3) it inhibits the ability to make free-standing membranes; (4) it can cause deformation in the shape of amorphous diamond structures when released from the substrate; and (5) it can cause the substrate to distort (to become bowed).
For a PLD film, the compressive stresses are generated continuously during film growth (e.g., the compressive stress remains roughly constant as a function of film thickness). D. R. McKenzie et al, Phys. Rev. Lett. 67, 773 (1991) suggests, based on thermodynamic arguments, that these high internal compressive film stresses are a necessary condition for the formation of carbon films with a high concentration of diamond-like, 4-fold coordinated atoms. Alternatively, J. Robertson, Diamond Relat. Mater. 3, 361 (1994), suggests that the high compressive stresses are simply the result of densification in the films due to implantation of energetic carbon species (10-100 eV) generated during the deposition process. Finally, M. Tamor, Applications of Diamond Films and Related Materials: Third International Conference, 1995, pg. 691 (1995), proposes that the high compressive stresses are a result of local dilatation within the film which occurs when a 4-fold coordinated carbon atom "relaxes" to a 3-fold coordination during film growth.